Project Summary: The proposed research will determine the mechanisms by which pulmonary surfactant reduces alveolar surface tension to exceptionally low levels. Functional surfactant is essential for maintaining the integrity of the lungs during normal breathing. Ventilation when surface tension is elevated injures the thin barrier that separates alveolar air from capillary blood. Our studies focus on two steps that pulmonary surfactant must accomplish to function effectively. Vesicles of pulmonary surfactant must first adsorb rapidly to the surface of the alveolar liquid and form an interfacial film. The hydrophobic surfactant protein SP-B is essential for that process. The adsorbed film must then convert to a structure that resists desorption when compressed by the decreasing surface area during exhalation. Our two specific aims each address one of these steps: Specific Aim #1: Determine how SP-B promotes adsorption. Specific Aim #2: Determine how films of pulmonary surfactant become resistant to desorption. Our studies are based on models that explain the available experimental data. Our experiments test specific hypotheses predicted by the models. The design of our studies emphasizes the structural alterations that cause the functional changes. We will use X-ray scattering to obtain the structural information. Functional experiments will test effects on both adsorption and desorption using captive bubbles. Studies will also determine the compositional dependence of structures observed in vitro. We will test whether surfactant function in situ in excised lungs and in vivo in hyperoxic rabbits require the same components. Our studies will provide the basis for understanding a fundamental aspect of pulmonary physiology that may be altered by any process that involves the alveolus. Our results will also yield the basis for the rational design of artificial surfactants that can extend surfactant-therapy from premature babies to adults.